Preheat oven to 350 degrees.
Bring a pot of water to a boil. Add broccoli and cook for 2 minutes. Drain.
In a large skillet melt butter over medium heat. Add onion, celery, and mushrooms. Cook until soft, about 5 minutes.
Add garlic powder and flour. Cook and stir for 1 minute.
Gradually add milk. Bring to a simmer. Simmer for 2 to 3 minutes to thicken.
Add salt, black pepper, cayenne pepper and 1 cup of shredded cheddar cheese. Remove from heat and stir until cheese is melted.
Add sour cream and mayonnaise and stir to mix.
Stir in rice and broccoli. Transfer to a greased 9X13-inch pan.
Sprinkle the remaining cheddar cheese on top.
In a bowl, mix together melted butter and Panko crumbs. Sprinkle on top of casserole. Bake for 30 to 35 minutes.